I believe it has ever since going Retina. It wasn't always presented up front, but it was an options.
OSX uses slightly different trick - it creates a framebuffer that is @2 scale and lets the hardware to scale it down. (iOS goes even further and has @3 scaling).
Which is the right approach - for example, one of the rMBP options is to have 1440x900 (2880x1800) resolution, which to the native 2560x1600 is the ratio 9/8. Nobody is going to track that 9-th pixel for each block of 8 pixels in software. Using hw scaler is simpler, cheaper and has smaller margin for errors.
Frankly, implementing fractional scaling is waste of time, resources and would produce software that is slow, buggy and bloated.
The fact that the framebuffer size is not the same as display resolution is known only to the driver and it is not something that is requested to be implemented for Linux.
Well whatever the case might be, whether it's kernel's fault, GNOME's fault, Wayland's fault or device driver fault, fact of matter is that one can easily get 1920x1200 resolution on Retina Macbook 15" with sharp font and images, and cannot on Fedora. This is why perhaps people are making "a fuss".
OS X CAN do fractional scaling as far as user is concerned (doesn't matter who scales it up or down), while Fedora cannot (with Wayland, at least).
It doesn't matter whether they know it or not, since they work transparently great with the scaling factor. In fact it's even better than having the software stack aware of it (simpler, less room for errors).
The end result is that fractional scaling resolutions works on OS X and not on Wayland (from what I read, haven't checked Wayland myself recently).
Thanks.